It is a common requirement, particularly for space satellite applications of communications, that each element of a multi-element antenna array, e.g. phased arrays, should be driven by a respective power unit, a single power amplifier. The amplified power signal for each antenna should be accurately controlled in amplitude and phase relative to the other antenna elements. It is common to employ solid state power amplifiers (SSPAs) for this purpose, but the amplifiers must meet stringent linearity, efficiency and gain/phase tracking performances required for operation in active antenna arrays.
The degree of gain compression, efficiency, and linearity of an amplifier can be optimised at a given RF output power by careful selection of the bias conditions. As the amplifier drive is reduced however, the amplifier backs off and the gain becomes less compressed resulting in degraded efficiency. The linearity improves but in many cases this is not an application requirement. When operating in active antenna arrays this results in some or all of the amplifiers running at less than optimum efficiency depending on traffic loading and beam pointing conditions (which may impose an amplitude ‘taper’ across the array, where amplifiers have a gradually reducing output amplitude across the array).
For the avoidance of doubt, terms used above have the following meaning:
Gain compression: may be defined, for example: a reduction in ‘differential’ or ‘slope’ gain caused by nonlinearity of the transfer characteristic of the amplifier. Gain compression may be understood as the degree of deviation from linear amplification as the amplifier approaches its saturation characteristic at maximum amplification.
Back off: the amount by which the input power and output power are reduced as the amplifier operating point moves along its operating characteristic away from a high power condition towards a lower power condition.
Efficiency: the ratio of RF output power for transmission to DC input power provided by a power supply.
Linearity: there are various measures of linearity as described herein, but a common measure is Carrier Intermodulation Ratio (C/I ratio—the ratio of power in the subject channel to power “leakage” in an adjacent channel of a multicarrier system).
The automatic adjusting of amplifier bias to minimise efficiency degradation with back off is disclosed for power amplifiers incorporating Silicon Bipolar Transistors in “Recent developments in solid state power amplifier technology and their applicability to third generation mobile space segment systems” pages 264-268, Fourth International Conference on Satellite Systems for Mobile Communications and Navigation; October 1988. Such technique is an analogue technique involving adjustment of the base emitter voltage of bipolar output transistors. Such technique could not be made to work with amplifiers employing Field Effect Transistors (FETs) and is therefore severely limited for amplifiers in current use.
Other more complex techniques are known—notably Chireix outphasing (LINC) amplifiers and Doherty amplifiers. These arrangements require splitting an input signal into two parallel amplification paths, and then subsequently combining the amplified signals. Such techniques are complicated to set up and are primarily intended to maximise the efficiency at a given back off where the amplifier has to demonstrate very high linearity.
None of the above techniques addresses the requirements of amplifiers operating in modern active array antennas. An essential pre-requisite for such amplifiers is that they must track one another in gain and transmission phase, and in many cases with the amplifiers being operated at different drive levels and temperatures.
In “L Band Power Amplifier Solutions for the INMARSAT Space Segment”, IEE Seminar on Microwave and RF Power Amplifiers, 7 Dec. 2000, D. Seymour, pages 6/1-6/6, a system is disclosed for controlling a solid state power amplifier (SSPA) in gain, gain slope and phase, so that these characteristics are accurately held constant and tracked relative to other SSPAs of an array of a large number of SSPAs. The system includes an Electronic Power Conditioner (EPC), which is a power supply adapted for use in space applications. A control ASIC receives an Amplifier Temperature signal and an Amplifier Input Power signal, and accesses digital compensation data held in an EEPROM for providing, in dependence upon the received signals, appropriate analogue control signals for control of gain, gain slope, and phase of the amplifier.